Composite plastic and metal matrix materials reinforced with high-modulus, high-strength filaments such as boron and silicon carbide are finding increased popularity in structural applications. In particular, these types of composites are useful where high strength and stiffness with accompanying low weight is desired.
The classical silicon carbide filament contains a refractory core, generally tungsten. The core is overcoated with silicon carbide. The overcoating is accomplished by means of a hydrogen reduction chemical vapor deposition process wherein gases containing sources of silicon and carbon are decomposed and silicon carbide coats the core. The thickness of the coating is directly related to the deposition time.
The most widely-used cores are tungsten. Cores of carbonaceous nature, such as graphite and carbon monofilament are being developed as carbonaceous cores in combination with silicon carbide coatings have exhibited greater strength and stiffness than similar coatings on tungsten cores. Accordingly, the trend has been to improve the quality and manufacturing techniques for silicon carbide on carbon filaments.
Of the tests used to evaluate the quality of a silicon carbide monofilament, two--the pull test and the bend test--are most significant. In the pull test, opposite ends of a length of silicon carbide filament are clamped within the jaws of a standard tensile tester and tension is applied till the filament ruptures.
In the bend test, a length of silicon carbide filament is bent around the surface of a circular cylinder or disc. The stress at the outer surface of a filament is inversely proportional to the bending radius formed by the cylinder. The maximum surface strength is determined by the smallest diameter loop the filament can withstand without rupturing.
Standard, or non-treated, 5.6 mil silicon carbide filament on a carbon core can be bent to a minimum diameter of about 9/16th of an inch. This computes to a maximum surface strength of about 650Ksi. The pull tests, such standard silicon carbide filaments have a tensile strength of about 350Ksi.
A buffer layer of oriented graphite between the core and silicon carbide coating had no beneficial effect.
Silicon carbide filament has been shown to be sensitive to surface abrasion which lowers its tensile strength. In order to improve this surface tensile strength and lessen the sensitivity to surface abrasion, a surface layer of carbon rich silicon carbide is applied to the silicon carbide coating during the deposition process. A surface treated 5.6 mil silicon carbide filament exhibited surface tensile strength, in the bend test, to values in the range of 1,400 to 1,600Ksi. In pull tests, these filaments still exhibited strength of about 350Ksi.
While a tremendous increase in surface strength was achieved by surface treatment, surface treatment did little for pull strength.
It is an object of the invention to provide an improved silicon carbon filament which affords greater reliability of obtaining a higher tensile strength than was available from prior silicon carbide filaments.
Another object of the invention is to provide a method for economically and reliably making improved silicon carbide filament.
In accordance with the invention, there is provided a silicon carbide filament containing a carbon core overcoated with a coating of silicon carbide with an inner surface layer of carbon rich silicon carbide. The coating may also contain an outer surface layer of carbon rich silicon carbide.
The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in conjunction with the accompanying drawings, in which:
It is hypothesized that silicon carbide is particularly sensitive to the presence of non-stoichiometric silicon carbide or impurities. I. T. Kendall, Journal of Chemical Physics, Vol. 21, pg. 821 (1953). Since both Kendall and K. Arnt & E. Hausmanne in Zeits Anorg Chem., Vol. 215, pg. 66 (1933) have found no evidence of non-stoichiometric silicon carbide, it is hypothesized that the excess carbon appears in the silicon carbide as an impurity. The properties of silicon carbide are particularly sensitive to the presence of impurities such as carbon.